Oxygen-enriched air and nitrogen-enriched air are required on enclosed vehicles for a wide range of applications. In aircraft, oxygen-enriched air, for example, is used for crew or passenger breathing, for starting the engine of emergency power units and for other aircraft services that require oxygen. Nitrogen-enriched air is used to pressurize and blanket fuel tanks with predominantly inert gas to reduce the risk of fuel tank fires. The need for substantially pure oxygen is especially important for military aircraft. In such aircraft, that operate at high altitudes and that encounter severe manuevering conditions, the crew oxygen system desires a gas of at least 95% oxygen.
Conventional methods of providing oxygen-enriched and nitrogen-enriched air on an enclosed vehicle include storing tanks of gas on the vehicle, and using the oxygen-enriched or nitrogen-enriched gas when needed. The enclosed vehicles have tanks of oxygen-enriched air, such as compressed oxygen-enriched air, compressed oxygen or liquid oxygen, stored on the vehicle. The tanks of gas require recharging after use. This conventional method is expensive, requires extensive recharging facilities and poses a hazard in having compressed gas tanks on the enclosed vehicle.
The disadvantages of the conventional system led investigators to develop on-board systems of generating oxygen-enriched and nitrogen-enriched gas streams that would eliminate the need for extensive compressed air, compressed oxygen or liquid oxygen storage on the enclosed vehicle. Such a system also would eliminate the need for extensive landbased recharging or regenerating stations, and would reduce the space and weight requirements of extensive on-board storage of oxygen or oxygen-enriched air. Preferably, the system and method would provide substantially pure oxygen, such as about 99% or greater oxygen, for the crew and passengers of an aircraft that operates at high altitude or for the crew of an enclosed military vehicle operating in a contaminated atmosphere.
However, these on-board gas enrichment systems possessed disadvantages in regard to size and weight requirements that are undesirable in an enclosed vehicle. Furthermore, the previously developed systems were capable of generating only either an oxygen-enriched gas or a nitrogen-enriched gas. In addition, the gas generating methods were inefficient, such that the enriched gas stream often included a relatively large concentration of an unwanted gas or a potentially harmful contaminant. Therefore, investigators have continually sought improved on-board gas generation systems to provide an adequate quantity of substantially pure breathing oxygen to the crew and passengers of an enclosed vehicle.
Therefore, on-board gas generating systems have been installed in enclosed vehicles, like aircraft and military vehicles, to simplify the logistics of maintaining stored enriched-gas systems. But the prior on-board gas generating systems were relatively inefficient in that an adequate quantity of substantially pure oxygen was not provided; were bulky and heavy because they required compressors or a series of on-board gas generating units to sufficiently separate the air and generate an adequate supply of oxygen; or required extensive maintenance, such as periodically replacing adsorbents, like zeolites, that preferentially adsorb oxygen over nitrogen. Therefore, in accordance with an important feature of the present invention, a ceramic oxygen separator is used to separate oxygen from air and generate both substantially pure oxygen and nitrogen-enriched air, while overcoming many of the disadvantages found in the prior on-board oxygen generating systems.
The present on-board gas generating system and method continuously provide a sufficient amount of substantially pure oxygen for common enclosed vehicles, like an aircraft, from any convenient air source. Generally, an aircraft has one or more primary engines that provide thrust for the aircraft and pressurized bleed air for the environmental control systems. Turbine-powered aircraft, like other complex aircraft, also require an auxiliary power unit to provide electrical and hydraulic energy, and bleed air, when the primary engine or engines of the aircraft are not in use, for example when the aircraft is on the ground. This bleed air includes a sufficient amount of oxygen for breathing and other purposes if the oxygen content of the bleed air can be separated from the other gaseous components of the air. The separation of oxygen from bleed air is especially important at high altitudes, for example above about 20,000 feet, where modern aircraft often the operate and where the density of the air is too low to provide sufficient breathing oxygen.
It is necessary therefore to equip an aircraft, or other enclosed vehicles, with an on-board oxygen generating system that is capable of operating independent of external conditions, like air density or contaminants present in the air, and that can provide a sufficient amount of oxygen for crew or passenger breathing in an emergency situation, such as in a contaminated atmosphere or at high altitudes. Ideally, such an oxygen generating system is compact, lightweight, reliable, easily maintained and requires no special handling, while providing a continuous and sufficient supply of substantially pure oxygen.
As previously stated, ambient air at a high altitude, or in a contaminated atmosphere, contains a sufficient amount of oxygen for breathing, But the oxygen must be isolated from the ambient air in a sufficient quantity and in a substantially pure form for breathing. Preferably, the substantially pure oxygen also could be stored for later use. Investigators therefore sought methods of isolating oxygen from the ambient air to meet the oxygen quality and quantity requirements with a compact and lightweight apparatus. Consequently, numerous methods and systems were deviced to separate a stream of air having an ambient concentration of oxygen and nitrogen into a usable supply of gas possessing an enhanced oxygen concentration and into a usable supply of gas possessing an enhanced nitrogen concentration.
For example, Manatt in U.S. Pat. No. 4,508,548 disclosed an air separation module based upon the differing permeabilites of oxygen gas and nitrogen gas through a hollow, permeable film. The method utilizes a pressure gradient to separate the oxygen from the nitrogen in air. However, the method provides only moderately oxygen-enriched air, i.e. 35-45% oxygen, whereas ambient air includes about 21% oxygen; and provides nitrogen-enriched air still containing about 9% oxygen. Manatt teaches that such oxygen-enriched air can be used in aircraft for breathing purposes, but no suggestion was made that such moderately oxygen-enriched air is suitable for breathing purposes at a high altitude or in a hostile environment.
In contrast, the on-board oxygen generation system and method of the present invention provide a substantially pure stream of compressed or uncompressed oxygen gas including about 99% or greater oxygen. Furthermore, the substantially pure stream of oxygen is essentially free of contaminants that may be present in the ambient air. The substantially pure oxygen can be used by the crew of military aircraft operating at high altitudes and by the crew of a military vehicle operating in a contaminated atmosphere because the present method and system selectively separate the oxygen from the ambient air, and essentially exclude contaminants present in the air, such as biological and chemical warfare agents.
Benedict, in U.S. Pat. No. 2,609,059, disclosed separating the components of a gas mixture that includes a readily condensable component. Benedict utilizes the differing diffusion velocities of the gaseous components through a diffusion screen, and a cooling column to condense a condensable component of the gas mixture. The condensable component of the gas mixture therefore is separated from the noncondensable, or less readily condensable, components of the gas mixture. The method of Benedict also teaches using several of the disclosed separation units in series to achieve an effective separation of components. Benedict does not teach or suggest the separation of a gas mixture including only noncondensable, or relatively noncondensable components, such as separating air into a substantially pure oxygen stream and a nitrogen-enriched stream. Furthermore, Benedict does not teach or suggest simultaneously separating and compressing a stream of substantially pure oxygen from air.
Garwin, in U.S. Pat. No. 3,250,080, disclosed the fractionation of a gas mixture including components that have a different diffusion rate through a semipermeable membrane. The disclosed method utilizes a pressure gradient to separate the gaseous components over a series of diffusion cells to eventually provide a relatively pure stream of a gaseous component. Garwin does not teach or suggest a method of providing substantially pure oxygen with a single oxygen separation device.
McDonald et al., in U.S. Pat. No. 4,560,394, disclosed an oxygen separation system utilizing a membrane that is relatively more permeable to oxygen than nitrogen. McDonald et al. teach that the oxygen-enriched gas stream can be used for increasing the oxygen content of air for breathing in an aircraft. McDonald et al., however, do not teach a method of providing substantially pure oxygen that can be used for breathing in an enclosed vehicle when ambient air is unsuitable, or contaminated, for breathing purposes. Furthermore, McDonald et al. do not teach or suggest a method of separating an air stream into a compressed oxygen-enriched stream and into a nitrogen-enriched gas stream.
Glenn et al., in U.S. Pat. No. 4,681,602, disclosed a method of separating bleed air into an oxygen-enriched gas. The Glenn et al. method utilizes at least two air separators to provide either a sufficiently oxygen-enriched gas stream including 90-95% oxygen or a sufficiently oxygen-depleted gas stream. Glenn et al. do not teach or suggest a gas generating system or method that provides a substantially pure and compressed oxygen gas stream and a sufficiently nitrogen-enriched gas stream by utilizing a single oxygen separator.
Fee et al., in U.S. Pat. No. 4,877,506, disclosed a ceramic oxygen separator having a particular corrugated configuration. Fee et al. teach that the corrugated oxygen separator can purify air in an enclosed space. Fee et al. do not teach or suggest the use of a ceramic separator on an enclosed vehicle to provide a compressed oxygen-enriched gas stream and nitrogen-enriched gas stream from ambient air.
Another type of oxygen generator is discussed in Aviation Week and Space Technology, pp. 56-57, (Feb. 3, 1980). This publication described an adsorption-desorption interaction between oxygen and a molecular sieve to provide breathing oxygen for an aircraft crew. The method utilizes a molecular sieve, such as a zeolite, to preferentially adsorb the oxygen in the bleed air over the nitrogen, and therefore store the oxygen for later use.
Therefore, in summary, methods and systems have been developed to separate a mixed gas stream, like air, into its component parts. The prior methods and systems utilized to separate an air stream into an oxygen-enriched stream and a nitrogen-enriched stream were heavy and bulky, or required a series of separators to provide an adequate amount of substantially pure oxygen for breathing at a high altitude or in a contaminated atmosphere. Other methods utilized tanks of compressed air or oxygen, or utilized an oxygen adsorbent. But such methods have the disadvantages of requiring storage tanks that are heavy and bulky, and depending upon landbased recharging and regenerating stations.
Therefore, the continuous generation of oxygen or oxygen-enriched air on-board an enclosed vehicle eliminates the need for recharging facilities at each base. Logistics are thereby simplified. Furthermore, until the system and method of the present invention, the prior methods also required an on-board compressor to compress the oxygen or oxygen-enriched gas stream if storage of the oxygen was desired. Therefore, in accordance with an important feature of the present invention, a substantially pure stream of oxygen gas, including about 99% or greater oxygen, is generated continuously on-board an enclosed vehicle from an atmospheric air source, such as bleed air, ram air or unpressurized ambient air. The method selectively separates the oxygen from the air source, thereby essentially excluding a contaminating or noxious agent in the air supply from the breathing oxygen. The generated stream of substantially pure oxygen can attain a pressure of up to about 10,000 psi (pounds per square inch), thereby eliminating the need for an on-board compressor to compress the substantially pure stream of oxygen. The substantially pure oxygen can be used as it is generated, or, if desired, can be stored for later use, such as breathing or to start and maintain combustion in an emergency power unit.